The trait of twinning rate has been shown to possess economic value to the dairy industry. The negative effects of increased twinning rate include increased dystocia, retained placenta, longer return-to-estrus intervals, an increased frequency of freemartins, reduced milk production and more frequent involuntary culling (Beerepoot et al. 1992; Niellen et al. 1989). It has been estimated that economic loss to the dairy industry is around $110/cow due to lost revenue and increased costs from producing twin calves instead of singles (Beerepoot et al. 1992; Eddy et al. 1991); after accounting for effects of inflation, the cost in current dollars is likely considerably higher. Reducing the incidence rate of twinning will therefore reduce this loss and increase profitability for the dairy industry.
Genetic progress for selection on twinning rate has been made (Gregory et al. 1997). Non-zero estimates of heritability for the trait have been observed, suggesting an additive genetic component. Estimates have ranged from 0.06%-21.6% depending on parity inclusion and type of model used in its estimation (Johanson et al. 2001; Ron et al. 1990; Karlsen et al. 2000). Nevertheless, the majority of evidence suggests twinning rate to be a lowly heritable trait, and therefore an excellent candidate for the use of marker assisted selection to aid in its genetic progress.
Various studies have identified QTL affecting twinning rate or ovulation rate in cattle (Blattman et al. 1996, Kappes et al. 2000, Kirkpatrick et al. 2000, Lien et al. 2000, Arias and Kirkpatrick, 2004, Gonda et al. 2004, Cruickshank et al. 2004, Cobanoglu et al. 2005). Linkage mapping has shown positive results for the existence of QTL segregating in the Holstein population. A drawback of linkage mapping is that confidence intervals remain broad (Andersson & Georges 2004). In many reports throughout the literature, analyses have attempted to refine the locations of QTL using additional markers or larger datasets; however, this has been accomplished with minimal success. Alternative tools or approaches are necessary to refine these locations.
Recent advances in technology have facilitated screening of the genome with markers at higher density than previously possible (Hardenbol et al. 2005; Thompson et al. 2007). An example of a proposed analysis method has been reported by Meuwissen & Goddard (2001). Their method predicts identity by descent (IBD) probabilities at a given location using the information from the marker genotypes, or haplotype, surrounding the given location. Haplotypes provide more information compared to single-markers when linkage disequilibrium (LD) is weak, consequently improving the power of QTL detection.
Twinning or ovulation rate QTL on bovine chromosome 5 (BTA5) have been identified in previous studies. Twinning rate QTL were detected between 55˜65 Mb (bovine genome assembly 3.1) on BTA5 in the Norwegian dairy cattle population (Lien et al. 2000) and the North American Holstein population (Cruickshank et al. 2004). An ovulation rate QTL was detected in the 40 cM (˜30 Mb) region of BTA5 (Kappes et al. 2000) in the USDA twinning population.
Though progress has been made, there remains a need for improved methods and genetic markers for predicting twinning potential for individual animals and within herds.